1. Field of the Invention
This invention relates to methods of controlling solids precipitation from geothermal brines, particularly to methods of preventing precipitation of soluble salts from supersaturated brine concentrates remaining after flashing the brine.
2. State of the Art
When extracted from the earth, hot aqueous geothermal brines contain many dissolved solids. Geothermal brines produce steam used to generate power after flashing, that is, reducing the temperature and pressure rapidly to produce steam and a brine concentrate. The rapid temperature reduction caused by flashing forces many dissolved solids in the brine to precipitate out of brine concentrate. Usually, the solids that precipitate from the brine concentrate are either sparingly soluble or virtually insoluble at standard temperature and pressure (STP, herein 20.degree. C. and one atmosphere). These sparingly soluble or insoluble solids tend to produce the bulk of the scale normally precipitated in geothermal operations.
However, salts that are normally considered to be quite soluble have recently started to cause precipitation problems as well. Some geothermal brines have recently been producing brine concentrates that have an increased concentration of dissolved salts. This phenomenon is particularly noticeable in brine produced from older formations. The salt concentration of, for example, chloride salts of sodium, potassium, and magnesium, may become supersaturated as the brine concentrate cools, causing the salts to precipitate. The pipes and other equipment in the geothermal facility then become caked not only with a refractory scale from the insoluble salts, but also from normally soluble salts as well.
Since precipitation of solids from aqueous solution is a result of exceeding a critical concentration, salt precipitation from the brine concentrate can be prevented by diluting the salt concentration by adding more water. If enough water can be obtained, this method works well. However, many geothermal resources are found in semi-arid or arid regions where fresh water is scarce, precluding its use for diluting geothermal brines. However, the liquid that condenses from steam used to generate power, usually referred to as condensate, provides another convenient source of fresh water for the operator of geothermal plants. Condensate is then a candidate for diluting the brine concentrate remaining after flashing.
If water, whatever its source, is added to the brine concentrate, the added water must not change the conditions of the brine concentrate so much that the entire brine treatment system must be overhauled. The systems in place are the result of exhaustive testing and optimization. They are therefore sensitive to the smallest change, which upsets the system. Water condensed from geothermal steam contains problem-causing impurities that flash from the brine with the steam. These problem-causing impurities limit the use of condensate as a diluant.
Both ammonia and hydrogen sulfide flash with water under reduced temperature and pressure. Thus when the flashed geothermal steam ultimately condenses, the liquid condensate tends to contain significant concentrations of ammonium sulfide. The pH of the ammonium sulfide-containing condensate causes a problem. The steam condensate is alkaline, especially compared to the acidic pH of the brine concentrate. The high pH of the condensate causes system upsets by precipitating pH sensitive components. One solution to the pH incompatibility problem, discussed at length in U.S. Pat. No. 4,978,457, issued to Gallup et al., hereby incorporated by reference in full, is to adjust the pH of the condensate to within about one pH unit of that of the brine concentrate. This solution is further described in U.S. Pat. Nos. 4,522,728, and 4,615,808 also issued to Gallup et al, both patents hereby incorporated by reference in full.
However, it has been found that merely acidifying the condensate does not solve all the problems associated with using the condensate to dilute the brine concentrate. Another problem is caused by the high sulfide ion concentration in the condensate. Metal sulfides do not precipitate from the brine concentrate in normal operation. However, contacting the metal ion-containing brine concentrate with an ammonium sulfide-containing condensate will produce metal sulfides. The nearly insoluble metal sulfides will then precipitate out of solution, causing system upsets. Therefore, sulfide concentration of the condensate can potentially cause a new problem by precipitating metal sulfides from the brine concentrates.
A seeming solution to the second problem is to contact the condensate with air to oxidize the sulfide ion. However, allowing the condensate to contact air to oxidize sulfide causes a further problem. When the condensate is left in a pond, or the like, a substantial concentration of oxygen dissolves in the condensate, causing the condensate to become very corrosive to the interior of the pipes in the geothermal plant. Such a corrosive solution is incompatible with an acceptably long service life in a geothermal plant.
In summary then, although condensate could advantageously be used to dilute a brine concentrate to prevent soluble salts from precipitating, before a geothermal plant operator can use condensate three problems must be overcome. First, the pH of the condensate, which is different from the brine concentrate, must be corrected. Second, the high concentration of sulfide ion in the condensate must be reduced. Third, the condensate must not be allowed to dissolve oxygen. All three problems must be solved before the operator can mix the condensate with the brine concentrate to produce an injection stream without causing system upsets. If the operator can produce an injection stream, he can then inject it into the earth, disposing of the brine concentrate, the condensate, and the dissolved salts
It would be advantageous to have a method of treating the condensate so it could be added to the brine concentrate. Then it could be added to reduce the concentration of soluble salts in the brine concentrate solution. Such a method would allow one to add the condensate to the brine without upsetting the system or corroding the pipes.